Hycanthone, lucanthone and niridazole, chemotherapeutic agents in the treatment of schistosomiasis, are attracting wide concern because of their potential carcinogenic and mutagenic hazards to treated individuals in the human population. Our previous studies have shown that hycanthone, lucanthone and their indazole analogs are mutagenic in Neurospora crassa. Niridazole is not mutagenic under the condition that conidia are treated. It is of interest to test the mutagenicity of niridazole in growing vegetative cultures of N. crassa and to determine the type of genetic alterations caused by hycanthone, lucanthone and their indazole analogs. The ad-3 test system of N. crassa is used in this study. Conidia from a genetically marked two-component heterokaryon are used for mutation induction experiments. Treated and untreated conidia are analyzed for the presence of ad-3 mutants by the direct method. The isolated mutants are characterized by complementation, dikaryon and trikaryon tests. The results indicate that niridazole is mutagenic only when the vegetative cultures of N. crassa are treated and that hycanthone, lucanthone and their indazole analogs appear to induce predominantly multilocus deletion mutations.